1. Field of the Invention
The invention refers to a method of increasing the volume of cut tobacco stems by impregnating the cut ribs at least with water, heating and drying the impregnated cut ribs in a pneumatic transport system by means of a gaseous heating and transport medium containing steam as well as to an apparatus for carrying out said method.
2. Description of the Prior Art
A plurality of methods are known in the case of which, for the purpose of drying tobacco and/or increasing the volume thereof, said tobacco is treated by being brought into contact with hot gases, such as air, air/steam or steam alone. In GB Pat. No. 875,684, for example, a method is described in the case of which the tobacco is dried by means of heated air. U.S. Pat. No. 3,357,436 describes an apparatus for drying tobacco in the case of which humid, heated air having a water content of at least 10% is used, the tobacco being treated in a pneumatic system at a temperature of from approx. 65.degree. C. to approx. 316.degree. C. The patent description discloses that the drying is carried out comparatively slowly using several drying towers and that the initial moisture content of the tobacco to be treated must not exceed 35% by weight. Accordingly, very low filling power increases of e.g. 2.3% are reported.
In the case of another type of method the tobacco is intensively moistened and is also exposed to a heating medium, which solely consists of hot air, in a heating zone. Such a method is described in German-Auslegeschrift No. 21 03 669. According to said method the intensively moistened and, consequently, swollen up tobacco is heated by means of a hot gas such that only the marginal area on the surface of a tobacco particle is dried to a considerable extent in comparison with the inner area thereof, whereby a solid consistency--as compared to the consistency of the inner area--and, consequently, stability of shape of the swollen up tobacco particle is to be achieved after a speedy cooling-down process. The drying process as such is carried out in a means in which the moistened tobacco is transported--and simultaneously vibrated--on a conveyor belt through individual drying chambers through which a stream of hot air passes.
In the case of this method the speed of the gaseous heating and drying medium is reduced to such an extent that the tobacco or tobacco rib particles can only be maintained in the suspension state. This has the effect that the drying time is several minutes; the large number of gas inlet points in the moving-bed base effects, on the one hand, a certain amount of swirl in the tobacco particle layer, but, on the other hand, this mode of operation results in channel formation and, consequently, in non-uniform expansion effects as well as in a varying final moisture content of the treated tobacco material.
U.S. Pat. No. 3,734,104 and German-Auslegeschrift No. 2 253 882, respectively, describe a method according to which moistened, cut tobacco ribs are dried at a temperature of from 121.degree. C. to 371.degree. C. for a period of from 0.3 to 3 seconds at the most by means of a gaseous heating medium containing vapour. In the case of this method the ribs are pneumatically transported at a very high speed and are dried from a moisture content of from 24 to 60% down to a moisture content of as little as 6%. After having been treated, the filling capacity of the ribs measured by means of a densimeter is increased up to 50%.
The same patent specification demonstrates by means of examples the influence of the final moisture content of treated cut ribs on the increase in volume. For example, the density of cut ribs dried to a moisture content of 13.8% exceeds the density of cut ribs dried to 6.0% by approx. 12% and the density of cut ribs dried to a moisture content of 14.5% exceeds the density of cut ribs dried to 6.0% by approx. 23%, i.e. according to the teaching of the above-mentioned patent specification the volume increasing effect increases as the final moisture content of the treated material decreases.
The essential disadvantages of such a method are:
The high rates of drying required for obtaining a favourable volume effect can--if the very short treatment times are observed--only be achieved by means of very high temperatures, this having the effect that the fire risk is increased and that a high amount of energy is consumed.
The cut ribs dried to a water content of less than 10% become very friable, whereby--in view of the high transport speeds--a high loss due to fragmentation and dust formation is caused.
The treated cut ribs must be remoistened to a moisture content of from 12 to 13% prior to further processing, whereby a considerable part of the obtained increase in volume can get lost, if no additional, in some cases rather complicated measures are taken.
As mentioned in German-Offenlegungsschrift No. 29 43 373, the tobacco material is even dried to a final moisture content of 3% in order to obtain a satisfactory increase in volume. The above-mentioned Offenlegungsschrift additionally describes a complicated and expensive method of remoistening the expanded tobacco, which is employed for the purpose of limiting the loss of effect, which would otherwise be caused, to a minimum.